Geoscience Reference
In-Depth Information
Trenberth and Dai (
2007
) recorded several decades of drought in south-east Asia
following the June 1991 Pinatubo eruption in the Philippines. They attributed the
drought to a regional weakening of the hydrological cycle caused by cooling that had
been engendered by the eruption. A growing body of evidence also now suggests a
causal relationship between historic eruptions and El Ni no events, resulting in sea
surface temperature anomalies of near global extent (Adams et al.,
2003
; de Silva,
2003
). Droughts are strongly controlled by changes in sea surface temperature (Lamb
and Peppler,
1992
; Williams and Balling,
1996
), so another possible mechanism for
prolonging the initial reduction in precipitation following an eruption (Parker et al.,
1996
; Trenberth and Dai,
2007
) is the cooling of the ocean surface, which can last for
many decades following the initial eruption (Gleckler et al.,
2006
).
Although low latitude volcanic eruptions appear to propagate more widely across
the globe, as with the historic eruptions of Tambora (1815), Krakatau (1883), Agung
(1963) and Pinatubo (1991), high latitude eruptions can also have a major impact
on the tropics. For example, the 1783-1784 Laki volcanic eruption in Iceland was
associated with significant cooling (
C) in the Northern Hemisphere during
the boreal summer of 1783, weakening of the African and Indian monsoon circulation,
reduced rainfall in Africa south of the Sahara and much-reduced Nile flow (Oman
et al.,
2006
). Likewise, the 1982 eruption of El Chichon was in part responsible for
severe drought in the Sahel. Oman et al. (
2006
) used the record of Nile flow to support
a date of 939 for the eruption of Eldgja volcano in Iceland, which was the largest
high-latitude volcanic eruption of the last 1,500 years.
−
1to
−
3
°
23.6 The 1968-1973 drought in the Sahel and Ethiopia
The
Sahel
(Arabic for 'shore' or 'border') refers to the semi-arid margins of the
southern Sahara, and after two decades of above-average rains, it experienced a severe
and prolonged drought starting in the late 1960s (
Figure 23.8
). Dwindling waterholes,
pasture depletion and dying herds forced many nomadic pastoralists living in this
region to move south in a desperate search for food and water for their animals and
for themselves. The impact of erratic rains on a nomadic Tuareg community in central
Niger, visited by the author in December 1974, is ably described by Bernus (
1974
)
and is forcing a progressive move towards a more sedentary way of life (Grove,
1993
). Elderly pastoral nomads and peasant farmers could still recall two earlier
droughts of comparable severity to the 1968-1973 drought during the last century
(Laya,
1975
; Salifou,
1975
), and historical records indicate that aridity became more
pronounced in the Sahel at the end of the 'Little Ice Age' of Europe more than a
century ago (Nicholson,
1976
; Nicholson,
1978
; Nicholson,
1980
;Nicholsonetal.,
2012
). Although the precise limits of the Little Ice Age are hard to define, the overall
period lasted from about 1450 to 1850 AD, with significant glacier advances from
1550 to 1700 AD and again during the 1830s and 1850s (Lamb,
1977
). Five cores
